1. Field of the Invention
The present invention relates to a communications system utilizing a TFO function.
2. Description of the Related Art
Recently, the mainstream of the mobile communications system field has been a digital mobile communications system for digitizing voice and data and transmitting the digitized voice and data. Since a mobile terminal is provided with a memory dial function, a call can also be originated using a phone number registered in the memory dial if recently a call has been originated from the mobile terminal.
In a mobile communications network, the most restricted resources are a radio section between a mobile terminal and a base station. To reduce a required bandwidth in a radio section, a voice encoding system for reducing the transmission rate of 64 kbps usually used in an ordinary telephone network to a low transmission rate of 9.6 kbps and the like, is used for voice transmission. In the case of a voice-encoder operation, both a mobile terminal and a fixed network side must have at least one kind of a voice encoder/decoder.
In the mobile terminal side, a voice-encoding function is embedded inside the mobile terminal, and in the fixed network side, a voice-encoding function is located in a base station or a mobile switching center.
In voice communications at the time of each mobile-terminal origination or mobile-terminal termination, a transcoder is connected to the fixed network side. The transcoder decodes voice signals (up-link direction) originated by a mobile terminal and encodes the voice signals (down-link direction) terminated by a mobile terminal. If one voice communicator party is a mobile terminal and the other party is, for example, a subscriber of a public telephone network (PSTN), this method enables a normal operation without any problem.
In the case of a mobile-terminal/mobile-terminal communications (MM connection) where the opposite party in a mobile-terminal origination is a mobile terminal, a mobile communications network includes a transcoder between an origination side mobile terminal and a mobile switching center, and a second transcoder is located between a called side mobile terminal and a mobile switching center (the same mobile switching center or a different one). For each of these transcoders, two transcoder units are connected in series for each MM connection communication, and thereby the voice-encoding/decoding operation of the transcoder degrades voice quality.
For this reason, a specification of tandem-free operation (TFO) is recommended in order to prevent tandem-encoding. (The TFO recommendation in 3GPP2 which specifies the technical specification of cdma2000 is “A.S 0004”, and it is “JP-3GB-A.S 0004” in TTC recommendations. The TFO recommendations specifying the technical specification of W-CDMA are “TS23. 053” and “TS28. 062”.) This TFO establishes synchronization between the first and second transcoders in a voice band. If they are synchronous, a TFO is performed (specifically, neither the first nor second transcoders are used). If they are asynchronous, the voice-encoding/decoding operation of the transcoder is performed (PCM mutual conversion between a low transmission rate and 64 kbps).
FIG. 1 shows a system configuration for implementing a TFO.
In FIG. 1, mobile terminals MS1 and MS2 are connected through a base station 1, a TRAU (transcoder and rate adapter) 1, a fixed network 10, a TRAU2 and a base station 2. The MS1 encodes a call-connection request using an encoder, and inputs the encoded request to the TRAU1 through the base station 1. The TRAU1 negotiates with the MS1 about a voice-encoding type, determines the voice-encoding type and outputs a connection request to the TRAU2 through a decoder. The TRAU2 receives signals from the TRAU1 in an encoder and outputs the call-connection request to the MS2 through the base station 2. The MS2 decodes the received call-connection request, negotiates with the TRAU2 about a voice-encoding type and determines a voice-encoding type between the TRAU2 and MS2. Then, the TRAU1 and TRAU2 judges whether the voice-encoding type between the MS1 and TRAU1 and the voice-encoding type between the MS2 and TRAU2 match (for example, whether the transmission rates are the same) through the encoder and decoder, respectively. If they match and are synchronous, the TRAU1 and TRAU2 enable voice signals from the MS1 and MS2 to bypass the encoder and decoder, respectively, and enable the MS1 and MS2 to communicate through an up TFO and down TFO, respectively. If the voice-encoding type on the MS1-side and the voice-encoding type on MS2-side do not match, the TFO described above is not performed, communications are established by converting the signals received from the MS into signals to be transmitted on the fixed network 10 using the encoder and by converting the signals received from the fixed network 10 into signals to be transferred to the MS using the decoder.
If the kind of a voice code supported by both mobile terminal and fixed network sides is a single type at the time of MM connection communications, it is strongly anticipated that the connection becomes the MM connection by a TFO. However, since the kind of voice code supported by mobile terminal and fixed network sides becomes multiple types in the future, it is considered that there is a low possibility that the communications may become MM connection communications by a TFO. This is because at the time of MM connection, the voice-encoding type on an origination side and the voice-encoding type of a termination side are independently determined and then the signals are synchronized in a voice band by a TFO.
FIG. 2 is a sequence chart showing a connection between mobile terminals.
It is assumed that a call connection request is transmitted from a mobile terminal MS1. The call connection request is transmitted from the mobile terminal MS1 to both a base station BTS and a base station control center BSC1. Here, the voice-encoding type (transmission rate and the like) on an origination side is determined between the mobile terminal MS1 and the base station BTS/base station control center BSC1. Then, the call connection request is transferred from the base station BTS/base station control center BSC1 to a mobile switching center MSC1, is switched and is transferred to a mobile switching center MSC2. Then, the call connection request is transferred from the mobile switching center MSC2 to the base station BTS/base station control center BSC2 on a termination side. Then, the call connection request is reported from the base station BTS/base station control center BSC2 to a mobile terminal MS2 on the termination side. At this moment, the voice-encoding type on the terminating side is determined between the mobile terminal MS2 and the base station BTS/base station control center BSC2. An attempt is made to synchronize the TRAU of the base station BTS/base station control center BSC1 on the origination side and the TRAU of the base station BTS/base station control center BSC2. If they are synchronous, a TFO is performed (both a decoder and an encoder are bypassed).
FIG. 3 shows the format of a switching office side signal in a PDC. FIG. 4 shows a TFO frame structure in a CDMA EVRC speech codec.
FIG. 3 shows a frame format in the case where a TFO is performed in a PDC, and it is judged whether a voice-encoding type on an origination side and a voice-encoding type on a termination side match and are synchronous, using bits F0 to F15 shown in FIG. 3. If it is judged that they are synchronous, information about a low bit rate (V.SELP) is superposed on a slashed part shown in FIG. 3 and necessary control information is exchanged.
In FIG. 4, a bit C5 is an embedded TFO message indicator bit and indicates whether a frame includes a message on a TFO. Although negotiation is made between a mobile terminal and a base station/base station control center prior to a TFO, this negotiation is made by embedding a voice-band signal in a part of Other C bits shown in FIG. 4 (1) and exchanging information. If a TFO is performed, information about a low bit is superposed on a part of Other D bits shown in FIG. 4 (2) and information needed for a TFO is transmitted/received.
FIG. 5 is a sequence chart showing an origination/termination connection in the prior art.
First, an origination message is transmitted from a mobile terminal MS1 to start communications. Upon receipt of this message, a base station BS1 transmits a reply message (base station ack order) to the mobile terminal. The base station BS1 also transmits a resource use permission request (complete L3info:CM service request) to a mobile switching center MSC-a. Upon receipt of this request, the mobile switching center MSC-a transmits an initial address message (IAM) to a mobile switching center MSC-b. In response to this message, the mobile switching center MSC-b transmits a paging request to a base station BS2. The request is transmitted to a mobile terminal MS2 through the base station BS2 as a page message. A “connect call U (ACM)” is also transmitted from the mobile switching center MSC-b to the mobile switching center MSC-a.
In response to the paging request, the mobile terminal MS2 returns a page response message to the base station BS2. The base station BS2 returns a paging response to the mobile switching center MSC-b. The base station BS2 returns the reply message (base station ack order) to the mobile terminal MS2.
Upon receipt of the resource use permission request, the mobile switching center MSC-a determines a codec to be used and transmits an assignment request to the base station BS1. Upon receipt of this request, the base station BS1 notifies the mobile terminal MS1 of the codec to be used by a channel assignment message. Then, the mobile terminal MS1 transmits a message declaring a channel to be used (Tch preamble) to the base station BS1. The base station BS1 returns a “BS ack order” to the mobile station MS1, and the mobile terminal MS1 returns an “MS ack order” to the base station BS1. Then, the base station BS1 transmits a service connection message (service connect message) to the mobile terminal MS1. In response to this, the mobile terminal MS1 returns a service connection completion message (service connect completion) to the base station BS1. Upon receipt of the service connect completion message, the base station BS1 transmits a resource assignment completion message (assignment complete) to the mobile switching center MSC-a. Then, the mobile switching center MSC-a transmits a ring-back tone (alert with info) to the mobile terminal MS1.
Upon receipt of an assignment request from the mobile switching center MSC-b, the mobile terminal MS2 also performs the same process described above, and determines a codec to be used in the mobile terminal MS2. Then, the base station BS2 transmits “alert with info” to the mobile terminal MS2, and the mobile terminal MS2 transmits an “MS ack order” to the base station BS2. Then, the mobile terminal MS2 transmits a “service connect message” to the base station BS2. Then, the base station BS2 returns a “BS ack order” to the mobile terminal MS2 and also transmits a “service connect message” to the mobile switching center MSC-b. The mobile switching center MSC-b transmits a reply message (ANM) to the mobile switching center MSC-a, and the mobile switching center MSC-a transmits a “service connect message” to the mobile terminal MS1.
Thus, the mobile terminals MS1 and MS2 are connected.
If a TFO is performed in the procedure described above, for example, the voice encoding on the origination side becomes an “encoding type A”. If the voice encoding on a termination side is determined to be an “encoding type B”, the signals cannot be synchronized in a voice band by a TFO even if synchronization is attempted. As a result, the first and second transcoders described earlier are connected and voice quality is degraded. (If both of the mobile terminals select the same encoding type, tandem encoding can be prevented since they can be synchronized by a TFO.)